Precipitated calcium carbonate (PCC) is used extensively as a filler in alkaline paper due to its low cost and ability to impart opacity, brightness and bulk. The development of internal sizing agents that are effective at neutral and alkaline pH values has made it possible to produce internally sized PCC filled paper, and the sizing agents widely used for this purpose are alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA). These sizing agents react with the cellulosic hydroxyl groups and they generally give a high initial sizing response at small added amounts. However, in recent years it has been experienced that the sizing response initially obtained with AKD or ASA is not maintained over time. This phenomena has been referred to as size reversion, i.e., the sized paper is within specification at the reel, but, upon ageing, the sizing response decreases to a level where it then remains constant. In addition, there have been examples of in specification sizing to result in a complete loss of sizing with time, termed fugitivity. The practical consequences of size reversion and fugitivity in PCC filled paper have been seen in terms of poor economics, converting and end-use problems.
In order to counter size reversion, the papermaker has had to increase the level of sizing agent added internally or, additionally, add a surface sizing agent. Although this has often resulted in retarding of sizing deterioration, the increased level of sizing agent used has been found to cause conversion and end-use problems. For example, increased levels of AKD has resulted in slippage problems and poor toner adhesion. During converting of certain grades of fine paper, for example Forms Bond, envelope, adding machine tape and some cut-size grades, precise control is necessary during high speed handling. Here, paper slippage has resulted in, for example, dropped folds, perforation misregister and reduced press speed.
The use of rosin dispersions for internal or stock sizing is well-known in the art. The rosin material is fixed to the cellulose fibers by precipitation with aluminum compounds. However, in order to be effective, sizing with rosin dispersions should be carried out at acidic pH values between 4 and 6. When the stock has a neutral or alkaline pH, sizing with rosin becomes erratic and very difficult to control. In addition, carbonate fillers present in the stock may interact with the components of the sizing system and adversely affect the sizing efficiency. Thus, the impact of alkalinity and calcium carbonate on aluminum and rosin chemistries has hitherto rendered it economically as well as technically disadvantageous to apply rosin dispersions to papermaking systems including PCC as a filler.
It is further known in the art to combine rosin and cellulose-reactive sizing agents in order to obtain a more widely useful sizing agent. EP-A1-74 544 discloses a method of sizing using cationic dispersions of dispersed particles of a cellulose-reactive sizing agent and dispersed particles of fortified rosin. U.S. Pat. No. 4,743,303 discloses a method of sizing employing anionic and cationic dispersions in which the dispersed particles contain a mixture of a cellulose-reactive sizing agent and rosin material in the form of rosin and fortified rosin. U.S. Pat. No. 4,816,073 discloses a method of sizing utilizing the above anionic and cationic dispersions which further contain a polyaluminum compound. EP-A2-333 368 discloses a paper sizing process in which a pre-blended composition containing rosin, optionally extended with an AKD emulsion, and an aluminum compound is added to papermaking stock containing chalk. However, none of the above publications mention sizing in the presence of PCC and there is no indication that there is a particular problem caused by including PCC as a filler in sized paper.
It is an object of this invention to provide a method of sizing which reduces or eliminates the problems associated with size reversion and fugitivity in sized paper and similar cellulose products containing precipitated calcium carbonate as a filler. It is a further object of this invention to provide an aqueous dispersion of sizing agents which can be used to effect sizing according to the method of the invention.
The objects of the invention are achieved by a method of sizing and an aqueous dispersion of sizing agents as further defined in the claims.